This invention relates to auxiliary power steering systems for automotive vehicles of the type in which a servomotor operating piston drivingly interconnects a steering spindle with a steering mechanism while turning of the steering spindle controls fluid powered movement of the piston between stroke limits established by bypass valve assemblies mounted in the piston.
Auxiliary power steering devices of the foregoing type are already known as disclosed, for example, in German patent publication No. OS 24 12 549 or in U.S. Pat. No. 3,896,703. In such an auxiliary power steering device, two rods anchored to the steering gear housing are received within bores of the operating piston. Each of the rods has a reduced cross-sectional area portion of triangular shape so that prior to the piston reaching its limit position, excess flow of pressurized fluid is conducted from the pressurized operating chamber to the opposing chamber in fluid communication with the exhaust port through which a return flow of fluid is conducted to a fluid reservoir tank. The foregoing rods therefore act as bypass valves causing depressurization of the active operating chamber as the piston approaches its limit position. Cut-off of fluid operating power therefore occurs to avoid any unintentional powered movement of the operating piston at its limit positions.
Another type of travel position limiting, power cutoff device is disclosed in German patent publication No. OS 14 55 939, wherein two ball valves disposed adjacent opposite axial ends of a common bore in the operating piston, normally block flow through the bore under the force of a single spring and the operating pressure in the active operating chamber. Whenever the operating piston approaches one or the other of its limit positions, one of the ball valves is unseated by a pin attached to the housing. A flow passage is thereby established from the steering pump through the inactive operating chamber to the exhaust return port connected to the fluid reservoir tank.
Single function valves for limiting operating pressure are also known as disclosed for example in prior copending application, Ser. No. 41,395, filed May 22, 1979, now U.S. Pat. No. 4,261,248, owned in common with the present application by the same assignee.
Each of the foregoing fluid power cutoff arrangements, has only one function. It is therefore an important object of the present invention to simplify the equipment associated with an auxiliary power steering system having fluid power cutoff means to limit the piston travel and a relief valve for limiting the maximum permissible pressure developed within the active operating chamber of the fluid servomotor during piston travel.